What is the rule for using 'a' or 'an' before words starting with 'e'?

What is the rule for using 'a' or 'an' before words starting with 'e'?

The rule for using 'a' or 'an' before words starting with 'e' is to use 'an' before words that have a vowel sound and 'a' before words that have a consonant sound.